Remote conferencing computer systems are already established. Such interactive systems allow participants in mutually disparate geographical locations to participate in a collaborative activity in real time. Generally each participant has access to a computer means that is networked with the computers of the other participants. A display device accessible to each participant acts as a ‘virtual whiteboard’, allowing the participants to submit and receive drawings and alternative graphical or other information. Each participant may have a copy of a common drawing on his or her virtual whiteboard, and can edit or otherwise amend that drawing, such ‘markups’ being immediately transmitted to the other participants to maintain in a consistent state the common drawing.